LONDON — Britain on Sunday lowered its terrorism threat level, a day after the police arrested a second man in connection with the bombing in a London subway station that left dozens of people injured.

The man, 21, was arrested late Saturday in Hounslow, a borough in West London, the Metropolitan Police said in a statement. The suspect, who was not identified, was detained under the Terrorism Act and was being held at a police station in South London, according to the statement.

The second arrest came the same day the police detained an 18-year-old man “on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism” in connection with the bombing.

That teenager was arrested in the southeastern county of Kent, in the departure area of the Port of Dover, where ferries leave for France, according to The Associated Press.

The police called it “a significant arrest,” and the suspect was later transferred to a police station in London.

Witnesses said the carriage was engulfed in flames, and dozens of passengers were trampled as they tried to escape. No one was killed, but at least 30 people suffered injuries, including burns and fractures. Several people were hospitalized.

After the bombing, Britain raised its terrorism threat level to “critical,” the highest level, which means another attack is “expected imminently.”

Home Secretary Amber Rudd announced on Sunday that the threat level had been downgraded to “severe,” meaning that an attack was highly likely. She urged the public to be vigilant.

Ms. Rudd said on Saturday that it was “much too early” to say whether those behind the attack had been known to the authorities. On Sunday, she also cast doubt on the Islamic State’s claim.

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Britain was hit by a terrorist attack on a Friday morning in September 2017, when a crude device exploded on a crowded London Underground train. Passengers described seeing a wall of fire.Published OnSept. 15, 2017CreditImage by Chris J Ratcliffe/Getty Images

“It is inevitable that so-called Islamic State, or Daesh, will reach in and claim responsibility,” she told the BBC. “We have no evidence to suggest that yet.”

The Metropolitan Police said, “At this stage we are keeping an open mind around whether more than one person is responsible for the attack, and we are still pursuing numerous lines of inquiry.”

The British Transport Police Firearms said on Twitter that it had placed extra officers on duty to patrol stations.

The police said on Sunday that they were searching two houses, one in Stanwell, a village in the county of Surrey, and another in Sunbury-on-Thames, also in Surrey.

About 12:30 p.m. local time, according to a local television report, police forensics teams also put up cordons in a section of West London and began searching a property there. Residents were not allowed to leave, and the “sizable” operation was connected to the second arrest.

Assistant Commissioner Mark Rowley said in a statement that with “the high pace and rapid progress of this investigation,” the police were “getting a greater understanding of the preparation of the device” used to bomb the subway.

On Saturday, officers began searching the house in Sunbury-on-Thames after evacuating surrounding buildings. Some residents were offered transportation to a local rugby club, and others were sent to the homes of relatives nearby.

The couple were appointed Members of the Order of the British Empire in 2009 for services to children and families.

Neighbors said the couple were staying with friends during the search.

The police said they were “working to support displaced residents and to get them back into their homes as soon as possible.”

Commissioner Rowley also said that as the workweek began, specialized teams of investigators would maintain a “strong visible presence throughout the capital and across the U.K.,” and that the public would “still see that high level of policing presence, some armed, some unarmed.”

David Moll contributed reporting from Hong Kong.

A version of this article appears in print on , on Page A10 of the New York edition with the headline: Police Arrest A 2nd Man In London Train Attack. Order Reprints | Today’s Paper | Subscribe